Large vehicles employ an air brake system which is based upon the positioning of brake linings against a brake drum by use of biasing springs. In order for the brake system to be released, air pressure is used to compress the biasing springs thereby releasing the brakes. A benefit of such a braking system is that if the vehicle loses power or otherwise has a failure in the braking system, air pressure is lost causing the biasing springs to engage the brake drums thereby locking the wheels to prevent further movement. Release of the brakes requires air pressure of at least 80 PSI with normal operation of the system requiring approximately 125 PSI.
It is common for a braking system to lose pressure due to wear of pneumatic components and their associated seals. The Society of Automotive Engineers acknowledges pressure loss with a recommendation that pressure loss does not exceed 1.5 PSI per minute. This loss is acceptable in most instances as the system replenishes the air supply by use of an air compressor mounted on the vehicle. When the vehicle is not operational, the pressure to the biasing springs is removed causing the brakes to engage in a parking brake mode. During non-operation, it is not uncommon for the air to slowly escape until the air reservoir has fallen below an operating pressure capable of biasing the springs in an open position for release of the brakes. Thus, when a vehicle having an air brake system is started it can take up to ten minutes for the air compressor to build sufficient air pressure in order to fill the braking system allowing for the release of the parking brakes.
The problem is that in many situations it has been found that such a delay can result in life threatening circumstances. For instance, emergency vehicles such as fire trucks must either constantly replenish worn air system lines or wait sometimes up to five or ten minutes for the vehicle to build sufficient pressure in order for the fire truck to be moved. Furthermore, mechanical air pumps require high RPM's requiring the operator to race the engine immediately upon start-up to fill the reservoir. This leads to early engine wear and adds to the confusion during an emergency.
Delay is unacceptable in an emergency. For this reason, emergency personnel are instructed, upon the notification of an alarm, for one operator to immediately start the vehicle while the remainder of the crew readies for the emergency situation. Alternatively, it has been found that emergency stations use a separate air compressor used by mechanics to maintain tire air pressure to supply a constant pressure to the vehicle's air supply system. The disadvantages of external air are obvious. If the external supply line is not disconnected before the vehicle is moved, the line may damage the air reservoir disabling the vehicle. In stations where ten or fifteen vehicles may be employed, the multiplicity of external supply lines leaves hoses on the floor which can easily lead to accidents.
Various teachings set forth elaborate braking systems for accommodating the air pressure system, yet none of the prior art addresses the problem with emergency vehicles and the need for an instant supply of pressure for proper operation.
U.S. Pat. No. 4,191,428 discloses a vehicle air brake system having a pressure holding valve which maintains the air chambers of the parking brakes at a predetermined pressure level even if the air brake system is operating at a pressure level lower than the spring biasing position. While this invention discloses the need to prevent air brake lockup while the vehicle is in operation, it does not address the need for maintaining reservoir pressures while the vehicle is not operational.
U.S. Pat. No. 5,172,958 discloses an air brake control system for use with truck trailers. A fully discharged system requires air pressure for operation in which an emergency glad hand supply line builds pressure. Valves in the trailer cooperate to route all the air directly to charge the spring brake chambers, by-passing the air reservoir allowing for biasing of the brake springs so that emergency movement of the vehicle may be achieved. This system operates for emergency situations of the vehicle itself and not the situation. By simply pressurizing up the brake biasing springs the vehicle may be moved; however it is unsafe for operation on the road as the reservoir has insufficient pressure for normal operation.
U.S. Pat. No. 5,286,095 discloses yet another improvement in an air brake system which discloses the use of a valve which couples to the reservoir allowing only a predetermined amount of air to enter the reservoir before the valve diverts the air directly to the brake system for use in compression of the biasing springs. This invention allows minimal pressurization of the system before operation of the vehicle, thereby allowing the trailer to be moved under emergency situations should the air reservoir be damaged or ruptured. Depending on the size of the vehicle, this may take minutes to achieve operation yet the reservoir may have insufficient air for proper operation.
DOT regulations 49 C.F.R .sctn.571.121 requires that reservoirs on air brake systems be protected against loss of air pressure due to failure or leakage in the system between the service reservoir and the source of air pressure by check valves and that each reservoir shall have a condensate drain that can be manually operated. The air compressor must be of sufficient capacity to increase air pressure in the supply line and surface reservoirs from 85 pounds per square inch to 100 pounds per square inch when the engine is operating at the vehicles manufactured maximum recommended RPM within a time, in seconds, determined by the actual reservoir capacity times 25 divided by the required reservoir capacity. No provisions are taught for control of air during non-operation.
Thus what is needed in the art is a low cost means of providing emergency vehicles with the ability to immediately release the parking brakes upon demand by maintaining sufficient pressure within the system for proper operation.